In the manufacture of sheet materials, it is well known that various sheet properties can be detected "on-line,", i.e., while a sheetmaking machine is operating. On-line measurement devices include, for example, ones that measure sheet properties such as basis weight, dry basis weight, moisture content, and thickness. Typically, such on-line devices employ sensors that periodically traverse, or scan, sheets in the cross direction (i.e., in the direction perpendicular to the direction of sheet travel). Depending upon the particular sheetmaking machine, cross-directional distances can range from about 100 inches to over 400 inches.
Known scanning sensors for use on sheetmaking machines include ones that measure basis weight by detecting the amount of radiation that a sheet absorbs from beams of infrared light or other radiation of known wavelength. In systems employing such sensors, radiation through a sheet is usually compared at two different bands of wavelengths, one of which is a measurement wavelength band and the other of which is a reference wavelength band. Although scanning sensors have numerous advantages, they also have some practical shortcomings. For example, the time required to make a cross-directional traverse may cause control delays, sometimes exceeding several minutes.
Because of the limitations of conventional scanning sensors, it has been proposed to mount a plurality of identical sensors at fixed cross-directional locations on sheetmaking machines to detect sheet properties. For example, U.S. Pat. No. 3,806,730 suggests that an on-line measuring system can include a set of stationary distributor tubes that distribute light from a single source onto the surface of a travelling sheet. According to the patent, the distributor tubes are aluminum pipes that have reflective interior surfaces. The system described in the patent further includes a similar set of receiver tubes which are mounted to receive and convey light which has been transmitted through the sheet. The receiver tubes carry the received light to detectors whose outputs, according to the patent, can be used for estimating the basis weight and moisture content of sheet material. The patent teaches that each distributor tube is associated with a particular receiver tube so that, for any pair of tubes, the total light transmission distance is the same. Accordingly, from a practical standpoint, the system in U.S. Pat. No. 3,806,730 requires that the light source and detector are situated at opposite edges of a sheet.